Working to change red to green

Having identified the bug that causes red needle cast (RNC), Scion researchers are now hopeful they will one day be able to recommend control measures to forest growers.

In May 2008, a mystery needle disease on the East Cape of the North Island was found during a routine forest check. Unfamiliar dark bands, or lesions, were noted on green needles. The needles turned red and could be detached easily, giving rise to the local name of red needle cast.

Affected needles were analysed by researchers at Scion and an unknown species of Phytophthora was isolated from them. In 2012, DNA sequencing matched it with Phytophthora pluvialis, a newly described species from Oregon, USA.

Phytophthora, which literally means 'plant destroyer', are associated with serious plant diseases worldwide, including the blight that caused the Irish potato famine. In New Zealand, Phytophthora species are responsible for kauri die-back, collar and crown rot in apples and disease in avocados and grapes.

Lindsay Bulman, forest pathologist and science leader of the Scion forest protection team, says the team was concerned about the disease when it first alerted stakeholders. "We didn't know how far it would spread or how it would behave in following years. We immediately made it a priority for research.

In partnership with the forest industry and in consultation with MPI, we started a programme to control the disease and to ensure the pathogen wasn't a risk to trading partners.

"As part of that, we needed to develop a chemical control method to manage outbreaks that was cost-effective and compliant with Forest Stewardship Council rules," he says.

Scion pest management research leader Carol Rolando says Agrifos 600 (potassium phosphite) was chosen as a candidate as it was already used for controlling Phytophthora diseases overseas and in New Zealand by the horticultural industry. Copper fungicides were also explored, as they were already widely used to manage dothistroma needle blight.

"In controlled trials, both phosphite and copper were shown to be effective against P. pluvialis. We had some concerns that the efficacy of copper would be compromised in the field when fully exposed to sunlight and rainfall, and, given the urgent need to find a control, we decided to focus on phosphite in our next set of experiments. We were very pleased when these showed that phosphite sprayed onto the foliage of young pines significantly reduced the formation of RNC lesions for up to eight months," she says.

"Together with Plant Protection Chemistry NZ, we also investigated ways of improving phosphite uptake using adjuvants, the additives that help sprays wet, stick to and penetrate the foliage. The best of these was an organosilicone blend, Du-Wett® (Etec Crop Solutions Ltd, Auckland). When applied at 0.2%, it increased phosphite uptake nine-fold compared to the active ingredient applied alone."

Orlando says the trial results show phosphite has great potential as an effective chemical control for RNC. One application could provide protection for up to a year.

"Now we need to get out into the field, confirm our initial results, determine the best dose rates, the best times to spray for RNC and how effective phosphite is at an operational scale. This is planned for the coming year."

Bulman says Phytophthora diseases remain a concern. A lack of variation in P. pluvialis DNA indicates that the organism is a recent arrival in the country, although exactly when and where it arrived are unknown. Reports of new Phytophthora species, diseases and changes in behaviour of established Phytophthora species are increasing worldwide.

"With ever-expanding global travel, transport and trade, it is likely other Phytophthora species will find their way to New Zealand. And it is possible that P. radiata will be susceptible to at least some of them," he says.

Genetics may provide a long-term solution. Resistance to disease seems to be a moderately heritable trait in pine trees and Scion and the Radiata Pine Breeding Company are working together to identify RNC-resistant germplasm to breed robust, disease-resistant trees.

Disease-resistance is just one part of a major six-year Scion-led research programme to address the threat that Phytophthora species pose to forestry, horticulture and natural ecosystems. It offers the hope that one day the only colour in our forests will be green.